1. Field of the Invention
This invention relates to an epoxy resin adhesive composition. More specifically, it relates to an epoxy resin adhesive composition having excellent adhesion flexibility, heat resistance, and chemical resistance, suitable for use in the production of a flexible printed circuits.
2. Background of the Invention
Flexible printed circuits have enormous development potential for use in huge new product areas where extreme thinness, highest density and volume manufacturability are important. A typical flexible printed circuit consists of three layers of material: a dielectric layer, a conductor layer and a coverlayer. An adhesive is the bonding medium between layers for creating a laminate of flexible printed circuit. The adhesive chosen is normally carefully matched to achieve the best mix of desirable properties for the laminate. The adhesive must be able to meet the demands for the manufacturing of the lamination and the processing of the flexible printed circuit.
There are many requirements of adhesive to meet the different demands that can be placed on a flexible printed circuit, for instance, should have good thermal properties, chemical resistance, bonding strength and flexibility. In addition, the adhesive must be easily processing.
Epoxy resins have excellent characteristics in thermal properties electrical properties, mechanical strength, and chemical resistance, and good bonding to copper foil and to polyimide film. They are used as an adhesive materials for printed circuits or flexible printed circuits. However, epoxy resins have a drawback of being brittle, resulting from their excessively high density of crosslinking while the resins are cured with amines, acid anhydrides and phenol compounds. The epoxy resin used as an adhesive for the flexible printed circuits should be a flexible compound, because brittle adhesive may cause a deadly damage in any bending occurrence.
A conventional method of reducing the crosslinking density to improve toughness is by using either of the epoxy resin itself or by the curing agent being compounding in excess of one another. However, since the molecular chain is terminated by the epoxy resin or by the curing agent positioned at the molecular terminal in the curing reaction, the thermal properties and chemical properties of epoxy resin will be greatly deteriorated.
Another approach is by using a rubber modified epoxy resin composition which may give the epoxy resin toughness without reducing the crosslinking density and, furthermore, only modest reductions in thermal properties.
Compounding a rubber into the epoxy resin composition may obtain a satisfied toughness without degrading the chemical resistance and bonding properties, however, the thermal properties will be degraded. The rubber compounds may be easily oxidized and deteriorated under either a high temperature laminating process or a high temperature soldering process. Thus some antioxidant compounds can be employed into the rubber-compounding epoxy resin composition to overcome said oxidation problem. In the present invention, a hindered piperidinyloxy compound, formula I, ##STR1##
is served as anti-oxidation agent for both of epoxy resins and rubber compounds. The hindered amine compound may stabilize the epoxy resins and rubber compounds while they are under a high temperature processing. It was found in the present invention that employing small amount of the hindered amine compounds into the epoxy resin composition can dramatically improve the peel strength of laminate for flexible printed circuits.